As is well known, transmitting picture information about moving pictures on telephone wires without any adaption of the information is very space-requiring. Therefore, a number of methods have been developed for compressing the information without deteriorating the image quality too much.
Differential pulse code modulation or DPCM coding is such a method of compressing the information. In this method the knowledge that there are only small differences between two successive images is utilized. In the receiver a predictor predicts what the next image will look like by means of the preceding image. A difference is formed and only the prediction error has to be transmitted to the receiver, which contains a similar predictor for reconstructing the original image.
In transform coding the image is divided into blocks of e.g. eight times eight picture elements or PEL. The image content is mathematically transformed into so called transform coefficients which are then quantized. The coefficients for gray areas are zero, according to this method, and do not have to be transmitted over the telephone wires.
Hybrid coding is a third method, which is a combination of DPCM and transform coding. According to this method the same difference is formed as in DPCM coding, which difference is then transformed and quantized. The information is then transmitted in an especially suitable form, since the image information is concentrated in the first coefficients in the block while the remaining coefficients are zero.
Then, the information can be coded with variable length coding, VLC. The coefficients or the words can take on values between e.g., -128 and +127, but the probability for the different values is concentrated around zero, i.e. the probability for the value zero is greater than for the values .+-.1 which is greater than the probability for the values .+-.2, etc. This is utilized in VLC by coding the most probable values such that they get the least number of bits, and values having lower probability get a greater number of bits. For the coding and the decoding, special schedules, so called code trees, are used. However, not all the coefficients in the block are coded, since it is known that the block ends by a continuous succession of zeros, but the last non-zero is detected and coded and then a character EOB (end of block) is transmitted, defining that the remainder of the block only consists of zeros. EOB is also part of the code tree.
The three methods above are described more closely in Televerket's technical periodical, Tele, vol. 91, No. 4, 1985, pages 1-7, and the periodical Elteknik, No. 14, 1986, pages 48-52.
A drawback in this previous method is that the same code tree is used for the last non-zero word, though it is known that it cannot be zero or EOB. According to the present invention it is realized that it would be efficient to use a second code tree, especially designed for the last non-zero word. This can then be constructed especially advantageously in view of the possible values for the last word and their probabilities. As a result, 1-2 bits per block can be saved in sending the same information, implying a saving of appr. 3%.